The present invention relates to a stator for a polyphase electric motor/generator and, particularly, to a stator for a three-phase motor/generator having three phase windings with each phase winding including two groups of coils electrically connected in parallel.
Three-phase induction motors are important and popular motors used for a variety of applications. Three-phase motors are popular because the horsepower rating of a three-phase induction motor is typically 167 percent more than a single-phase induction motor having the same weight. An exemplary use for a three-phase motor is using the motor as a starter motor for an internal combustion engine. The starter motor assists the internal combustion engine during engine starting until the engine can sufficiently operate without the assistance of the starter motor. The internal combustion engine can be an engine for a lawn mower, tractor, automobile, power-generation system, or the like.
One of the problems with a three-phase induction starter motor of the prior art is having the motor generate enough torque to start or xe2x80x9cturn overxe2x80x9d the engine. The problem arises because the power source for the starter motor is typically a twelve volt direct current (DC) battery. By capping the power source at twelve volts, the starter motor of the prior art typically cannot generate enough torque by itself to turn over the engine. One solution to this problem is to provide gearing between the starter motor and a crankshaft of the internal combustion engine. The gear ratio of the gears between the motor and the crankshaft is designed to be a ratio sufficient to allow the motor to start the engine. However, the gears are subject to wear and, therefore, have a limited operational life. Accordingly, there is a need for an improved three-phase electric motor that is capable of generating enough torque to start an internal combustion engine without the use of gears.
The invention provides a polyphase electric motor/generator and a controller. The polyphase motor is typically a three-phase motor/generator having a rotor and a stator. The rotor is interconnected with a drive shaft of the engine such that when the rotor rotates the drive shaft also rotates. The stator includes a core having slots for receiving electrical wire. The stator further includes three phase windings. The three phase windings include wire that is wound in the slots of the core and are electrically connected to the controller. The controller provides a substantially alternating current (AC) three-phase signal to the phase windings resulting in a magnetic field being induced within the core. The interaction of the induced magnetic field with a rotor magnetic field causes the rotor to rotate, which in turn causes the drive shaft to rotate.
The amount of torque a three-phase electric motor generates is dependent upon the amount of electric current flowing in the phase windings. The flow of electric current in the phase windings induces the magnetic field within the stator core for interaction with the rotor magnetic field, resulting in the rotation of the rotor. Assuming everything else is equal, the larger the current in the phase windings, the stronger the magnetic field within the stator core and, consequently, the greater the amount of torque that is generated by the rotor.
One way to increase current flow in the phase windings is to increase the voltage applied by the power source. However, if the voltage of the power source is fixed, such as is the case with an engine having a twelve-volt DC battery, then this is not a practical solution. An alternative way to increase current flow in the phase windings is to use the stator of the invention. The stator of the invention reduces the impedance of the phase windings as seen from the power source. Reducing the impedance of the phase windings increases the amount of current flowing to the phase windings and, therefore, increases the amount of current flow in the phase windings.
Accordingly, a stator of the invention provides a stator core having a plurality of slots that receives electrical wire. The stator further includes a first phase winding wound on the stator including first and second wires electrically connected in parallel. The first wire forms a first group of coils having a first pattern, and the second wire forms a second group of coils having a second pattern. For example, the first wire may form four coils according to a first pattern where the first coil is wound clockwise, the second coil is wound counter-clockwise, the third coil is wound clockwise, and the fourth coil is wound counter-clockwise. Furthermore, for example, the second wire may form four coils according to a second pattern where the first coil is wound counter-clockwise, the second coil is wound clockwise, the third coil is wound counter-clockwise, and the fourth coil is wound clockwise. Thus, the first and second groups of coils have different winding patterns. The stator further includes a second phase winding wound on the stator including third and fourth wires electrically connected in parallel. The third wire forms a third group of coils having the first pattern and the fourth wire forms a fourth group of coils having the second pattern. The stator further includes a third phase winding wound on the stator comprising fifth and sixth wires electrically connected in parallel. The fifth wire forms a fifth group of coils having the first pattern and the sixth wire forms a sixth group of coils having the second pattern. Of course, each pattern could be extended to include additional coils.
The stator of the invention further provides that the first phase winding is wound such that an end of the first wire and an end of the second wire are disposed in the same slot and are electrically connected together, the second phase winding is wound such that an end of the third wire and an end of the fourth wire are disposed in the same slot and are electrically connected together, and the third phase winding is wound such that an end of the fifth wire and an end of the sixth wire are disposed in the same slot and are electrically connected together. Additionally, the invention further provides that the remaining ends of the first, second, third, fourth, fifth and sixth wires are electrically connected together.
By connecting the first group of coils in parallel with the second group of coils, the resultant impedance as seen from the power source is reduced in half when compared to connecting the first and second group of coils in series. Similarly, by connecting the third group of coils in parallel with the fourth group of coils, the resultant impedance as seen from the power source is reduced in half when compared to connecting the third and fourth group of coils in series. In addition, by connecting the fifth group of coils in parallel with the sixth group of coils, the resultant impedance as seen from the power source is reduced in half when compared to connecting the fifth and sixth group of coils in series. Thus, the overall impedance of the stator windings as seen from the power source is substantially reduced by the parallel connection. Reducing the overall impedance increases the amount of current flowing to the phase windings and, therefore, the overall torque of the motor is increased.
One of the potential drawbacks to increasing the current flowing to the stator is that the amount of heat being generated by the windings increases. However, another advantage of electrically connecting two groups of coils for each phase winding in parallel is that the current flowing to the stator splits between the two groups of coils. Splitting the current flow results in less heat being generated by the phase windings when compared to connecting two groups of coils for each phase winding in series.
The stator of the invention further provides that each coil has one or more turns. Assuming that the wire for each phase winding has the same cross-sectional area (e.g., if the wire for each phase winding is round, then each wire will have the same gauge), increasing the number of turns for each coil increases the impedance for the coil. Moreover, it should be apparent that reducing the number of turns for each coil reduces the impedance of each winding. Assuming that a constant voltage is applied to the windings, reducing the impedance increases the amount of current flow within the winding. However, increasing the amount of current flow while reducing the number of turns for each coil results in more heat being generated. Thus, reducing the number of turns for each coil from (x) turns to (xxe2x88x921) turns (e.g., from three turns to two turns) may result in too large of a temperature increase. Too large of a temperature increase may result in damage to the motor.
Another advantage of connecting the group of coils in parallel results in the xe2x80x9chalf-integerxe2x80x9d winding. If the number of turns for each coil is an odd number and each phase is connected in parallel, then, as seen from the power source, each coil appears to have a xe2x80x9chalf-integerxe2x80x9d winding. For example, if the first and second group of coils are connected in parallel and each coil has three turns, then, as seen from the power source, each coil has the electrical equivalent of one and one-half turns.
In a second aspect of the invention, a stator for a three-phase motor includes first, second, and third phase windings wound on the stator. Each phase winding includes coils forming at least four poles, where the number of poles is represented by the number (m), and (m) is an even number. The coils forming the poles are divided into two groups, the first group includes the coils for poles one to (m/2) and the second group includes the coils for poles ((m/2)+1) to (m). The coils forming the odd-numbered poles of the first group and the even-numbered poles of the second group are wound in a first direction (e.g., clockwise). The coils forming the even-numbered poles of the first group and the odd-numbered poles of the second group are wound in a second direction opposite the first direction (e.g., counter-clockwise). The advantage of the just described winding arrangement is that, for each phase winding, one end of a first wire forming the first group of coils and one end of a second wire forming the second group of coils are disposed next to each other in the same slot. The two wires disposed next to each other can be easily connected to the controller without using xe2x80x9cjumper wiresxe2x80x9d. In other words, for each phase winding, there is no need to bridge a wire between the two disposed-together ends before connecting the wires to the controller. Removing the bridge reduces the complexity of the phase windings, reduces the number of required connections between the first and second coils of each phase winding, and reduces the cost of manufacturing the stator of the invention.
The invention further includes a method of winding a stator for a three-phase motor. The method includes the steps of providing a core comprising a plurality of slots. The method further includes winding a first phase winding having first and second wires. The winding of the first phase winding includes the steps of placing one end of the first wire in a first slot, and winding the first wire on the core by a first pattern to form a first group of coils. The first group of coils forms at least two poles, where the number of poles is represented by the number (n) and the poles are numbered from 1 to (n). The winding of the first phase winding further includes winding the second wire on the core by a second pattern different than the first pattern to form a second group of coils. The second group of coils forms (n) poles and the poles are numbered from 1 to (n). The winding of the second wire results in an end of the second wire being disposed in the first slot.
The method further includes winding a second phase winding having third and fourth wires. The winding of the second phase winding includes the steps of placing one end of the third wire in a second slot, and winding the third wire on the core by the first pattern to form a third group of coils. The third group of coils forms (n) poles and the poles are numbered from 1 to (n). The winding of the second phase winding further includes winding the fourth wire on the core by the second pattern to form a fourth group of coils. The fourth group of coils forms (n) poles and the poles are numbered from 1 to (n). The winding of the fourth wire results in an end of the fourth wire being disposed in the second slot.
The method further includes winding a third phase winding having fifth and sixth wires. The winding of the third phase winding includes the steps of placing one end of the fifth wire in a third slot, and winding the fifth wire on the core by the first pattern to form a fifth group of coils. The fifth group of coils forms (n) poles and the poles are numbered from 1 to (n). The winding of the third phase winding further includes winding the sixth wire on the core by the second pattern to form a sixth group of coils. The sixth group of coils forms (n) poles and the poles are numbered from 1 to (n). The winding of the sixth wire results in an end of the sixth wire being disposed in the third slot.
The method further includes repeating the steps of winding the first phase winding, winding the second phase winding, and winding the third phase winding for a second set of each phase windings. By winding more than one set of phase windings, a user can increase the effective cross-sectional area of wire for each phase winding. For example, if two identical sets of phase windings are wound on the stator, then the effective cross-sectional area for each phase is twice the cross-sectional area if only one set of phase windings is wound on the stator. Increasing the effective cross-sectional area allows for more current to flow through each phase and allows each phase winding to generate less heat.
Other features and advantages of the invention will become apparent by consideration of the detailed description and accompanying drawings.